fix: Replace the question flow with a menu that turns components on and off

- Menu detects what is on and applies only the changes: SteamOS
  conversion, SteamOS theme, Steam Deck/Machine icons, single user mode,
  and Steam Machine support (only on Fremont hardware)
- Everything is reversible: KDE settings go through an undo journal
  (lib/state.sh), system files are restored from their backups
- Steam Machine support: LED driver, udev rule giving the user the LED
  files, steamos-manager for Steam's hardware settings; OpenRGB dropped
- Edit Plasma files only while plasmashell is stopped so it can't
  overwrite them
- README and AGENTS.md describe the menu, the component model and the
  test environment

lib/menu.sh

@@ -0,0 +1,118 @@
+#!/bin/bash
+# Interactive menu: detects which components are on, lets the user pick what
+# they want, then turns components on or off to match.
+# Sourced by setup-gamescope-boot.sh; not meant to be run on its own.
+
+# Menu order. Components are turned on in this order and off in reverse;
+# gaming must come first (single user builds on it).
+COMPONENTS=(gaming theme glyphs single machine)
+
+declare -A LABEL=(
+    [gaming]="SteamOS conversion: boot into gaming mode, Steam on the desktop"
+    [theme]="Install SteamOS theme: Vapor look, dark mode, SteamOS taskbar"
+    [glyphs]="Install Steam Deck/Machine icons: Deck button icons in gaming mode"
+    [single]="Single user mode: no password, lock screen or log out (SDDM)"
+    [machine]="Steam Machine support: LED bar driver, hardware settings in Steam"
+)
+declare -A CURRENT WANTED
+
+component_available() {
+    [[ "$1" != machine ]] || machine_available
+}
+
+detect_components() {
+    local c any=false
+    for c in "${COMPONENTS[@]}"; do
+        component_available "$c" || continue
+        if "${c}_status"; then CURRENT[$c]=1; any=true; else CURRENT[$c]=0; fi
+        WANTED[$c]=${CURRENT[$c]}
+    done
+    # First run: preselect the full SteamOS experience.
+    if [[ "$any" == false ]]; then
+        for c in "${COMPONENTS[@]}"; do
+            component_available "$c" && WANTED[$c]=1
+        done
+    fi
+}
+
+toggle_component() {
+    local c="$1"
+    WANTED[$c]=$(( 1 - WANTED[$c] ))
+    # Single user mode only makes sense on top of the SteamOS conversion.
+    if [[ "$c" == single && "${WANTED[single]}" == 1 ]]; then WANTED[gaming]=1; fi
+    if [[ "$c" == gaming && "${WANTED[gaming]}" == 0 ]]; then WANTED[single]=0; fi
+}
+
+show_menu() {
+    local i=0 c now want
+    echo
+    printf "  ${c_bold}%-3s %-6s %-6s %s${c_reset}\n" "#" "Now" "Want" "Component"
+    MENU_ITEMS=()
+    for c in "${COMPONENTS[@]}"; do
+        component_available "$c" || continue
+        i=$((i + 1)); MENU_ITEMS[$i]=$c
+        now="off"; [[ "${CURRENT[$c]}" == 1 ]] && now="${c_green}on${c_reset} "
+        want="[ ]"; [[ "${WANTED[$c]}" == 1 ]] && want="[x]"
+        printf "  %-3s %-6b %-6s %s\n" "$i" "$now  " "$want" "${LABEL[$c]}"
+    done
+    echo
+}
+
+run_menu() {
+    # Sets REAPPLY. Returns 1 if the user quit.
+    REAPPLY=false
+    local reply
+    while true; do
+        show_menu
+        read -rp "$(echo -e "${c_bold}Type a number to toggle, Enter to continue, a = also re-apply what's on, q = quit:${c_reset} ")" reply
+        case "$reply" in
+            "") return 0 ;;
+            a|A) REAPPLY=true; return 0 ;;
+            q|Q) return 1 ;;
+            *)
+                if [[ "$reply" =~ ^[0-9]+$ && -n "${MENU_ITEMS[$reply]:-}" ]]; then
+                    toggle_component "${MENU_ITEMS[$reply]}"
+                else
+                    warn "Unknown choice: $reply"
+                fi
+                ;;
+        esac
+    done
+}
+
+plan_changes() {
+    # Fills TO_DISABLE (reverse order) and TO_ENABLE. Switching single user
+    # on or off moves gaming mode to the other login manager, so gaming
+    # mode is re-applied then too.
+    TO_DISABLE=(); TO_ENABLE=()
+    local c i
+    for (( i = ${#COMPONENTS[@]} - 1; i >= 0; i-- )); do
+        c=${COMPONENTS[$i]}
+        component_available "$c" || continue
+        [[ "${CURRENT[$c]}" == 1 && "${WANTED[$c]}" == 0 ]] && TO_DISABLE+=("$c")
+    done
+    for c in "${COMPONENTS[@]}"; do
+        component_available "$c" || continue
+        [[ "${WANTED[$c]}" == 1 ]] || continue
+        if [[ "${CURRENT[$c]}" == 0 || "$REAPPLY" == true ]] ||
+            [[ "$c" == gaming && "${CURRENT[single]}" != "${WANTED[single]}" ]]; then
+            TO_ENABLE+=("$c")
+        fi
+    done
+}
+
+apply_changes() {
+    local c failed=()
+    LOGIN_MANAGER=plasmalogin
+    [[ "${WANTED[single]}" == 1 ]] && LOGIN_MANAGER=sddm
+
+    for c in "${TO_DISABLE[@]}"; do
+        echo; echo -e "${c_bold}Turning off: ${LABEL[$c]}${c_reset}"
+        "${c}_disable" || failed+=("$c")
+    done
+    for c in "${TO_ENABLE[@]}"; do
+        echo; echo -e "${c_bold}Turning on: ${LABEL[$c]}${c_reset}"
+        "${c}_enable" || failed+=("$c")
+    done
+    FAILED=("${failed[@]}")
+}